Originally Posted by OrangeCrush
Are you saying that if my outboard were a 9hp, I would want a flatter propeller so that it could run at higher RPMs, but since my outboard has more torque at low RPMs, I could get the same thrust with less fuel?
Thats exactly what I'm saying. The smaller engine would need to run at higher rpm to achieve the same power output. But running a prop slower is always prefered (on a displacement
hull) because it increases prop efficiency...or reduces "slip". And running slower is also nice and quiet, reduces engine wear, etc.
I've been researching this topic because I have a repowered boat, a pearson 30
, and although the engine never strains at all, I can't go very fast even at full throttle. The newer volvo diesel
just doesn't to the rpm of the old atomic 4, so even with a bigger prop I now have, I never achieve anything near hull speed
. I think I need to address the reduction gear. First step...figure out what I've got!
Anyway, if your boat is running well, then you made a good choice. If you happen across a used 9x9 for cheap
, it might be worth trying it out, just to satisfy your (ie my) curiousity.
My previous boat was a paceship 29 with an atomic 4. The standard 2 blade
had been replaced with a 3 blade
, high pitch prop. I got almost max speed as soon as I put it in gear, and had to keep the idle speed high so as not to stall it. And it would not rev up hardly any more. It was not a good situation. Quite the opposite to my current
Anyway, props are not an exact science. With efficiencies of around 50%, there is lots of room for improvement.
In general, the slower the engine, the bigger the prop, and the higher the efficiency. The new electric
engines can run very slow with nice big props and achieve much higher efficiency than internal combustion engine setups. They do this because of the limited battery
power available for most electrics. With gasoline engines, the opposite is true....gasoline holds a tremendous amount of power, and its easy to bring gallons and gallons with you. At least that was the thinking back when gas was cheap